title: Decay kinetics of benzophenone triplets and corresponding free radicals in soft and rigid polymers studied by laser flash photolysis.
abstract: The kinetics of transients formed under photoexcitation of benzophenone (B) dissolved in three different polymers was studied by ns laser flash photolysis. These polymers were the soft rubbers poly (ethylene-co-butylene) (EB), polystyrene block-poly(ethylene-ran-butylene)-block-polystyrene (SEBS) and hard polystyrene (PS). We monitored the decay kinetics of triplet state (3)B(*) and of ketyl radicals BH(●). We observed exponential decay of (3)B(*) and two-stage decay kinetics of BH(●) in EB. The first stage is a fast cage recombination of a radical pair (BH(●), radical of polymer R(●)). The second slow stage of BH(●) decay follows the second-order law with a relatively high rate constant, which corresponds to recombination of BH(●) in a homogeneous liquid with a viscosity of only ~0.1 P (about five times of 2-propanol viscosity). Application of a magnetic field (MF) of 0.2 T leads to deceleration of both stages of BH(●) decay in EB by approximately 20%. Decay kinetics of both transients were observed in SEBS. There was no MF effect on BH(●) decay in SEBS. We only observed (3)B(*) in PS. Decay kinetics of (3)B(*) in this case were described as polychromatic dispersive first-order kinetics. We discuss the effects of polymer structure on transient kinetics and the MF effect.
authors.affiliation: Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, Moscow, Russia; Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, Russia.
authors.forename: Peter P
authors.initials: PP
authors.lastname: Levin
authors.forename: Alexei F
authors.initials: AF
authors.lastname: Efremkin
authors.forename: Natalie B
authors.initials: NB
authors.lastname: Sultimova
authors.forename: Valery V
authors.initials: VV
authors.lastname: Kasparov
authors.forename: Igor V
authors.initials: IV
authors.lastname: Khudyakov
mesh_terms: D001577:Benzophenones / Q000737:chemistry*; D005609:Free Radicals / Q000737:chemistry; D007700:Kinetics; D007834:Lasers; D011108:Polymers / Q000737:chemistry*; D014783:Viscosity
chemical_list: D001577:Benzophenones; D005609:Free Radicals; D011108:Polymers